As a background art in the present technical field, JP-A-6-70173 (Patent Document 1) exists, for example. In this patent publication, the following description has been given: “[Object] The occurrence of a ringing in a reproduced image is prevented. [Configuration] In an edge judgment unit 22, it is judged whether or not an edge is present from the state of an image obtained by decoding an encoded image for each block basis. Then, in a plane approximation unit 32, concerning each block which has the edge, the plane approximation is made with respect to domains positioned on both sides of the edge, thereby creating an approximate image. Moreover, this approximate image is DCT-transformed, thereby being transformed into the approximate image in a frequency domain. Furthermore, this frequency-domain approximate image is supplied to a calculation unit 38, where this image is added to the decoded image in the frequency domain with a predetermined internal-splitting ratio associated therewith. Namely, in the calculation unit 38, a high-frequency domain, which is lost in the quantization processing, is added to the decoded image. The lack of this high-frequency domain is a cause for the above-described ringing. Accordingly, the addition processing in the calculation unit 38 allows the prevention of the ringing in the reproduced image.”